From a subsequent interview with James Heenan on methodology:
How do you monitor human rights in a context like DPRK?
We rely on first-hand testimonies from victims and witnesses: the thousands of people who have left the DPRK for other countries. We also use information published by the Government of the DPRK, as well as other open-source data such as satellite images. We use other sources, such as information from civil society organizations, to corroborate what we hear. For this report, the Office interviewed 314 victims and witnesses who left the country after 2014. We located these people with the help of our partners, including civil society.
So 'civil society' organisations (meaning political think tanks, USAID fronts etc) come and say "oh yeah, we work with some people who escaped North Korea in the last ten years, want us to hook you up with the some testimony".
So they interview the people provided by these groups (who are also often providing them money, housing, travel documentation in exchange for giving these testimonies) who tell them whatever they think they want to hear.
And then guys like James go, 'well it sounds kind of crazy but since we can't go to North Korea to verify it, I guess it must just be true! How terrible.'